The present invention relates to a magnetic field stabilizer in a nuclear magnetic resonance spectrograph (hereinafter referred to as “NMR spectrograph”) and a method of stabilizing an NMR magnetic field with the stabilizer.
NMR spectrographs are being used to measure various specimens including organic materials, solid materials and biological macromolecules. As described in a non-patent document 1 (Gerhard Wider, Technical aspect of NMR spectroscopy with biological macromolecules and studies of hydration in solution, Progress in Nuclear Magnetic Resonance Spectroscopy, vol. 32, p 225, 1998”), an apparatus of this kind measures a resonance frequency of a specimen placed in a static magnetic field and it is required that this static magnetic field be stable and have a rate of change of 1 ppb/hr or less with respect to time. To produce a stable static magnetic field, most of the existing NMR spectrographs are equipped with a superconducting magnet. Even in a superconducting magnet, however, it is virtually impossible to achieve a stability of 1 ppb/hr. A lock device for stabilizing a static magnetic field by feedback control is important in enabling NMR spectrographs to deliver the desired performance. With the development of studies on proteins requiring long-time measurement processes, the importance of such a lock device is increasing.
A non-patent document 2 (Benedict W. Bangerter, Field/frequency lock monitor for signal averaging with high resolution NMR sepectrometers, Review of Scientific Instruments, vol. 46, p 617, 1975) discloses a technique used for the lock device. An alternating-current magnetic field having a radio frequency is radiated from a lock transmitter to a specimen placed in a magnet for use in an NMR apparatus to excite a magnetization vector in the specimen. The motion of the excited magnetization vector is received through an antenna for detection and is detected by using a receiving circuit. A component called a dispersion signal (SD) is obtained by adjusting the phase of a reference signal used at the time of detection.
The dispersion signal SD is multiplied by a predetermined proportion constant G and the result of this multiplication is delivered to a lock power supply. The lock power supply causes a current proportional to G×SD (correction signal SC) to flow through a coil provided around the specimen (hereinafter referred to as “lock coil”) to form a correction magnetic field proportional to the correction signal SC around the specimen. The proportion constant G is determined so that the dispersion SD obtained after adding the correction magnetic field is close to zero, thereby stabilizing the static magnetic field.
As described above, the technique disclosed in non-patent document 2 is characterized by stabilizing the static magnetic field by controlling the current flowing through the lock coil, and the lock corrector can be easily implemented, for example, by using only an analog circuit. The above-described lock detector is also capable of obtaining an absorption signal SA having a 90° phase difference from the dispersion signal SD, but this signal is not used for computation of the correction signal SC. FIG. 5 shows an example of the ideal dispersion signal SD and the absorption signal SA. A detailed description of the dispersion signal SD and the absorption signal SA will be made below.
A patent document 1 (JP-B-2504666) discloses a technique for the lock device, which differs from that disclosed in non-patent document 2 in that the absorption signal SA is used together with the dispersion signal SD to compute the correction signal SC output from the lock corrector. The correction signal SC in patent document 1 is a weighted combination of SD/SA and (1/SA)(dSD/dt).
According to patent document 1, SD/SA is perfectly proportional to a frequency offset DF=ω−ω0 due to a disturbance in the static magnetic field when the frequency of the disturbance is low, and the proportion constant is 1/T2. T2 is a time period relating to the rate of attenuation of the detection signal VD and called a lateral relaxation time. According to patent document 1, T2 can also be obtained in the control loop and DF=(1/T2)(SD/SA) can therefore be computed and used as the correction signal SC in the lock control section.
According to patent document 1, the frequency offset DF is also equal to (1/T2)×(SD/SA)+(1/SA)×(dSD/dt). This equation is accurate and has the advantage of being unrelated to any condition and the advantage of including a noise component. Therefore, the technique disclosed in patent document 1 is suitable for use in a case where the disturbance amplitude is comparatively large.
The technique disclosed in patent document 1 has an advantage over the technique disclosed in non-patent document 2 in that the magnetic field can be returned to a stable value by a smaller number of control loops in comparison with the technique because of the proportional relationship between the frequency offset DF and the correction signal SC and good monotonicity.
The technique disclosed in non-patent document 2 still has a problem in terms of monotonicity of the correction signal. As is apparent from the graph of FIG. 5 showing the dispersion signal SD, the dispersion signal SD is divided into three regions having boundaries corresponding to the maximum and minimum values of the dispersion signal SD, and the sign of the change rate dSC/dDF is reversed between adjacent two of the regions. Accordingly, a plurality of frequency offsets DF are associated with one dispersion signal SD. Since the correction signal SC in non-patent document 2 is G×SD, the correction signal apparently lacks monotonicity. Therefore, it is difficult to obtain the desired constant G and a number of control loops are required to return the static magnetic field to a stable value, resulting in a reduction in control speed.
The technique disclosed in patent document 1 increases the control speed by solving the monotonicity problem with the technique disclosed in non-patent document 2. However, since division by the absorption signal SA is introduced, there is a possibility of the correction signal SC becoming unstable when the frequency offset is increased and when the absorption signal becomes smaller. This is because when the magnitude of the absorption signal SA becomes approximately equal to the magnitude of noise, the sign of the correction signal SC can be reversed by the influence of noise. Also, the extent of variation appeared in the correction signal SC due to an error a in receiving phase is larger than that in the case of the technique disclosed in non-patent document 2. Accordingly, the range of frequency offset DF controllable by using the technique disclosed in non-patent document 1 is narrower than that in the case of the technique disclosed in non-patent document 2. The technique disclosed in patent document 1 is advantageous in terms of monotonicity but has a problem that noise and instability with respect to an error in receiving phase are increased and a problem that the controllable range is reduced.
It is an object of the present invention to provide a high-performance NMR apparatus by implementing a lock device having improved monotonicity without reducing the control range.